The principles of design, construction, and operation of dual spin satellites is well-known in the art. Generally speaking, dual spin satellites are spacecrafts which are generally comprised of a gyroscopic spinning body, oftentimes referred to as a rotor, rotatably coupled by a despin motor and bearing assembly to a relatively stationary body, oftentimes referred to as a despun platform. Although the platform is referred to as despun, it may be rotating with respect to the rotor. The despun platform carries the payload, e.g., scientific instruments, communications equipment, surveillance equipment, antenna(s), or the like. Oftentimes, the spin axis of the spacecraft is the principal axis of minimum moment of inertia, which is inherently unstable. Consequently, internal energy dissipation, e.g., due to structural flexing, fuel depletion, fuel sloshing, etc., will cause nutation of the satellite, which is a coning motion of the bearing or spin axis about the total angular momentum vector, which is fixed in inertial space in accordance with the law of conservation of angular momentum, in the absence of externally-applied torques, e.g., due to solar radiation pressure and/or gravitational field gradients. However, it should be understood that nutation caused by internal torquing between various components of the spacecraft can result in an exchange of momentum therebetween, without changing the total angular momentum vector of the spacecraft. If not checked, the nutational motion becomes a tumbling motion.
Accordingly, active or passive nutational damping means must be provided to damp out nutation of the spacecraft about its spin axis. A platform-mounted passive nutation damping device is disclosed in U.S. Pat. No. 3,442,468, issued to Iorillo, whose teachings are herein incorporated by reference. A motor active nutation damping system is disclosed in U.S. Pat. No. 4,096,427, issued to Rosen et al., whose teachings are herein incorporated by reference. In the latter damping system, a closed loop servo feedback control system is utilized to apply corrective torques by means of controlling the speed of the despin motor, in order to maintain the desired spin axis orientation.
Additionally, in order to provide for spacecraft attitude control and stationkeeping, the rotor is generally provided with axial and radial jets and/or thrusters which are controllably fired in order to precess the spin axis in response to both instantaneous and gradual changes of the orbital inclination of the spin axis, e.g., due to solar pressure, gravitational anomalies, e.g., solar and lunar gravitational perturbations, and other external torques acting in both the latitudinal and longitudinal direction of motion of the spacecraft as it orbits the earth. The various types of spacecraft stationkeeping and attitude control systems need not be mentioned here, since they are so well-known in the art. Of course, the firing of the jets and/or thrusters necessitates the expenditure of fuel or propellant which is normally stored in vessels or tanks housed by the rotor.
It is also well-known that dual spin satellites which are fixed in a highly inclined earth orbit must be inverted on a regular, periodic basis, e.g., semi-annually, in order to avoid thermal overloading of the payload and other on-board hardware, and system components. More particularly, when the sun angle geometry becomes thermally adverse, the rotor-mounted jets are controllably fired to precess the spin axis by a total of 180.degree., to thereby invert the orientation of the dual spin satellite so as to prevent direct exposure to the sun. This is the predominant, presently available technique for accomplishing this spacecraft orientation inversion maneuver.
As will be readily appreciated by those skilled in the pertinent art, it would be highly advantageous to have available an essentially passive method of accomplishing this spacecraft orientation inversion maneuver, which does not require the expenditure of fuel, instead of the presently available method described above, for at least the following several reasons. First of all, the useful life of the spacecraft can be significantly extended, since the amount of fuel required for these inversion maneuvers is typically 1/3 or so of the total fuel supply of a dual spin spacecraft fixed in a highly-inclined orbit. Thus, the conserved fuel can be used for other purposes, such as stationkeeping and attitude control, which control is usually necessary to maintain useful or optimum operation of the dual-spin spacecraft for its intended mission. Secondly, all or a portion of the spacecraft launch weight normally allocated to the propellant budget for inversion maneuvers can be used instead to increase the useful payload weight. Thirdly, all or a portion of the launch costs attributable to the weight of the propellant normally utilized for inversion maneuvers can be saved. This cost savings can be quite significant since the launch cost per pound is very high, e.g., on the order of $20,000 per pound. It is believed that the overall cost savings attributable to conserved fuel is on the order of $2-$4 million. Fourthly, a passive inversion technique can be utilized to invert the orientation of satellites which have already been deployed (i.e., which are already in orbit), even satellites which are low on or out of fuel and thus, heretofore believed to be at or near the end of their useful lives, since without fuel, they can not have their orientation inverted, thereby resulting in destructive thermal overloading of the payload carried by and/or system components of the satellite. Fifthly, with a passive inversion technique, there will not be perturbations to the spacecraft orbit and/or attitude due to firing of jets, as are occasioned by the presently available inversion technique. These perturbations, if not eliminated, can disrupt or interrupt the continuity of operation of the satellite for its intended purpose, as well as require consumption of yet additional fuel for corrective attitude control maneuvers necessitated by these perturbations.
The present invention constitutes such a highly advantageous, essentially passive method for inverting the orientation of a dual spin spacecraft disposed in a highly-inclined orbit.